A new lab study has found that the H5N1 avian influenza virus can remain infectious in raw milk for more than a day at room temperature and over a week when refrigerated, raising significant concerns about the potential for zoonotic transmission through unpasteurized dairy products.
Published May 28 on the preprint server medRxiv, the non-peer-reviewed research was conducted by a team of UK scientists who studied the stability of H5N1 highly pathogenic avian influenza virus in raw cow and sheep milk. The study’s findings come amid growing concern about the virus’s spread in U.S. dairy herds since late 2023.
“High viral titres were detected in milk from infected cows, raising concerns about onwards human infections,” the authors wrote. “Although pasteurisation was shown to effectively inactivate influenza viruses in milk, unpasteurised milk still poses a risk of infection, both from occupational exposure in dairies and from the consumption of raw milk.”
Since its emergence in U.S. dairy cattle, H5N1 has been detected in more than a thousand cattle herds, various mammal species, dozens of dairy workers, and several individuals with no direct contact with infected animals.
To model potential exposure scenarios, the researchers incubated the virus in pasteurized cow milk at room temperature and at 4°C (39.2°F), simulating both ambient dairy conditions and refrigerated storage. They also ran tests on sheep’s milk using a lab strain of avian influenza.
“Although the survival of influenza viruses in milk was variable, we consistently found that under laboratory conditions substantial viral infectivity remained over periods when people might reasonably be exposed to infected milk—for over a day at room temperature and for more than 7 days when refrigerated,” the authors wrote.
“Our results highlight the zoonotic risk of H5N1 HPAIV in raw milk from infected animals and reinforce the importance of taking measures to mitigate this risk,” they added. “Our experiments aimed to model the ‘worst case scenario’ for the persistence of viral infectivity in milk and should be seen as providing an upper-bound estimate for viral survival under real-world conditions.”
The study, which is awaiting peer review, was supported by multiple UK research agencies including the Medical Research Council, the Biotechnology and Biological Sciences Research Council, and the Department for Environment, Food and Rural Affairs.
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